I currently run 2 Facebook groups and a forum, so have a fair bit of experience on this subject. You may end up getting more traffic through a Facebook selling group, but it takes an incredibly long time to build the numbers up. The quick way of getting the traffic/advertising is to let everyone from anywhere join, and it becomes another international selling page. Sticking to UK people only and it will build very slowly - it's taken 2 years to get just under 600 members on my UK Z3 Owners page (and a few overseas people were let in to build the group quicker).
Also being UK only means that people will miss out on potential overseas sales or purchases. The current Z3 selling page was started by a Brit, it's just there is a larger market in the US for parts. That's why it has a higher percentage of Americans selling on it.
But whatever happens, good luck with the venture.